A cyclotron accelerates charged particles (ions) in an outward spiraling orbit from an ion source located near a central axis to an outer radius at which the ions are extracted from the cyclotron. An early classical cyclotron is disclosed in U.S. Pat. No. 1,948,384 (inventor: Ernest O. Lawrence). In the classical cyclotron, ions are introduced into the acceleration chamber, which is evacuated, from any of a variety of sources (e.g., emitted from a heated filament or from bombarded lithium or discharged from a hot cathode). The ion is accelerated in the cyclotron chamber by a pair of electrodes, wherein the electrodes provide a high-frequency alternating or oscillating electric potential difference to cumulatively increase the speed of the ion as it travels in a substantially circular orbit of increasing radius in the chamber. The orbit of the accelerating ion is in resonance or is synchronized with oscillations in the electric accelerating field(s) to repeatedly accelerate the ion at successive half revolutions.
Specifically, the ion, when positioned between the electrodes, is attracted to the interior of the electrode that has a charge at that moment that is opposite to the charge of the ion; and the ion gains velocity from the charge attraction. The shift in the electric potential of each electrode shapes the substantially circular orbit of the ion. As the electric potentials of the electrodes are reversed, the ion is then accelerated into the interior of the other electrode; and the cycle is repeated. As the ion gradually spirals outward, the velocity of the ion increases proportionally to the increase in radius of its orbit, until the ion is eventually deflected into a collector channel to allow the ion to deviate outwardly from the magnetic field and to be extracted from the cyclotron.
The orbital pathway of each ion is further governed by a magnetic field generated by two poles on opposite sides of the electrodes. The poles produce a substantially uniform magnetic field with field lines extending transversely to the electrodes and normal to the plane of the electric field between the electrodes to provide weak-focusing to maintain the accelerating ions in or near the median acceleration plane of the chamber (i.e., providing vertical stability). A modern version of a classical cyclotron is described in U.S. Ser. No. 12/951,968, filed 22 Nov. 2010 (T. Antaya, inventor).
In addition to classical cyclotrons, current classes of cyclotrons include synchrocyclotrons and isochronous cyclotrons. Modern cyclotrons are primarily of the isochronous cyclotron type.
Like classical cyclotrons, synchrocyclotrons feature a magnetic field that decreases with increasing radius and is shaped to provide weak focusing. However, while the electrodes are operated at a fixed frequency in classical cyclotrons, the frequency of the applied electric field in a synchrocyclotron is adjusted as the particles are accelerated to account for relativistic increases in particle mass at increasing velocities at increasing radii. Synchrocyclotrons are also characterized in that they can be very compact, and their size can shrink almost cubically with increases in the magnitude of the magnetic field generated between the poles. High-field synchrocyclotrons are described in U.S. Pat. No. 7,541,905, issued to inventor Timothy Antaya, and U.S. Pat. No. 7,656,258, issued to Timothy Antaya, et al.
Like classical cyclotrons, the acceleration frequency in an isochronous cyclotron is fixed. Unlike the radially decreasing magnetic field in a classical cyclotron, however, the magnetic field in an isochronous cyclotron increases with radius to compensate for relativity. And unlike the weak focusing provided by the magnetic field in a classical cyclotron, an azimuthally varying magnetic field component is derived from contoured iron flutter pole pieces having a sector periodicity to provide an axial restoring force as ions are accelerated. Some isochronous cyclotrons use superconducting magnet technology, in which superconducting coils magnetize iron poles that provide the guiding and focusing fields for ion acceleration.
The magnetic field at the edge of a cyclotron is generally unsuitable for acceleration, so the beam reaches full energy before the edge field is encountered, though the beam then passes through the edge field as it is extracted from the cyclotron. The longer the beam takes to traverse the edge, the more the beam quality is affected. In addition, some asymmetric field elements are included in the chamber design to separate the extracted beam from the internal orbits and direct the beam into the extraction path. These asymmetric field elements may be magnetic or electric; electric field elements are more common, though the electric field strengths required are large, and these large field requirements tend to make the electrical field elements unreliable. Hence, beam extraction is one of the main challenges of cyclotron design. Even after careful design and implementation of ion introduction and beam acceleration, proper extraction of the ion beam promotes good beam quality. Effective ion beam extraction and good beam quality is particularly advantageous for applications where the beam will be used for patient treatment, as inadequate beam quality (emittance) can result in relatively large unintended radiation (from the beam striking part of the beam chamber or other surfaces).
The extraction problem is aggravated in compact high-field cyclotrons, as for a given energy gain per turn, the spatial difference between consecutive ion orbits is small compared with those in larger, lower-field cyclotrons, thereby making beam extraction at a particular orbit more challenging.